A spark plug is used for ignition for an internal combustion engine. A conventional spark plug is comprised of: a center electrode; an insulator having an axial bore for receiving the center electrode therein; and a metal shell surrounding and holding the insulator in a radial direction thereof. One end of a ground electrode is joined to the metal shell and the other end of the ground electrode forms a spark discharge gap with a front end portion of the center electrode. The spark discharge gap serves as a firing portion, and an air-fuel mixture is ignited by spark discharge.
When a firing portion of the spark plug is formed so as to project into a combustion chamber, the firing portion is kept as far away from an inner wall face of the combustion chamber which causes a flame quenching, thereby controlling a growth of flame kernel and improving ignitability. However, when the length of the ground electrode is extended in the axis direction, so that the firing portion projects into inside of the combustion chamber, a dimension to a joint portion with the metal shell, which serves as a base point of heat conduction of the ground electrode, is relatively long. As a result, thermal conductivity of the ground electrode diminishes and heat conduction property thereof deteriorates. Therefore, a failure, such as an oxidization of the entire front end side of the ground electrode, or a melting and damage of the ground electrode are likely to occur. Further, as the ground electrode is lengthened, endurance thereof over vibration, shock or the like tends to deteriorate.
Japanese Patent Application Laid-Open (kokai) No. S62-82684 discloses a spark plug in which a cylindrical portion projecting into inside of the combustion chamber from a mounting portion of the internal combustion engine is formed on a front end side of the metal shell. When the spark plug has such a configuration, wherein one end of the ground electrode is joined to the front end side of the cylindrical portion, the firing portion can project into inside of the combustion chamber without changing the length of the ground electrode in the axis direction. Compared to the case where the ground electrode is lengthened, the above-configuration can improve the ignitability of the spark plug without deterioration in thermal conductivity of the ground electrode and in endurance over vibration or the like by way of forming the firing portion to project into inside of the combustion chamber.